Final answer:
The Earth's temperature is regulated by the balance of incoming solar energy and outgoing infrared radiation. The Sun is the main source of energy, and the Earth maintains its temperature by radiating energy back to space in accordance with the Stefan-Boltzmann law.
Step-by-step explanation:
The main source of energy entering the Earth system is radiation from the Sun, which is predominantly in the form of ultraviolet rays, visible light, and infrared radiation generated by nuclear fusion.
Earth maintains a relatively constant temperature due to the balance of incoming solar energy and outgoing energy, which is emitted back into space as infrared radiation.
The planet is in thermodynamic equilibrium, since the energy absorbed from solar radiation is essentially balanced by the energy Earth radiates back into space, following the Stefan-Boltzmann law, which describes the power radiated from a body based on its temperature.
While the Earth reflects some of the solar energy it receives, it also produces some waste heat as a consequence of human activities and natural processes. This waste heat does not accumulate indefinitely because the Earth radiates it back into space as infrared radiation.
Earth's temperature is a balancing act; when it absorbs more energy than it radiates, it warms up, and when it radiates more energy than it absorbs, it cools down. Factors that influence this energy balance include natural and human-induced changes to the Earth's atmosphere and surface.